Quick answer: Current is the flow of charge in amperes, potential difference is the energy per coulomb in volts, and resistance in ohms is the opposition to current. They are linked by V = I × R. I-V graphs show that resistors are ohmic, while lamps, diodes, thermistors and LDRs are not.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE P2, IGCSE 4.3
What are current, potential difference and resistance?
In a circuit three quantities work together. The battery pushes charge, the charge flows round the loop, and the wires and components make it harder or easier for the charge to pass.
Current (I) is the rate of flow of charge, measured in amperes (A). Potential difference (V) is the energy transferred to or from each coulomb of charge, measured in volts (V): 1 V = 1 J/C. Resistance (R) is how strongly a component opposes the current, measured in ohms (Ω).
V = I × R
V = potential difference (V), I = current (A), R = resistance (Ω). Rearranged: I = V ÷ R and R = V ÷ I. Energy transferred: W = Q × V, with W in joules and Q in coulombs.
The ammeter measures the current in the loop, so it is in series. The voltmeter measures the energy per coulomb across a component, so it connects in parallel. Divide the voltmeter reading by the ammeter reading to get the resistance.
Aarav in Pune measures 2.4 V across a resistor while the ammeter reads 0.080 A. Calculate its resistance.
R = V ÷ I = 2.4 ÷ 0.080 = 30 Ω
A motorbike battery in Lahore moves 45 C of charge through its lamps at 12 V. Calculate the energy transferred.
W = Q × V = 45 × 12 = 540 J
What does Ohm’s law say?
Ohm’s law: for a component at constant temperature, the current is directly proportional to the potential difference. Such a component is called ohmic, and its resistance is constant.
A longer wire has more resistance because charge has further to go, and a thicker wire has less resistance because there is more room for charge to flow. Doubling the length doubles the resistance, and doubling the cross-sectional area halves it.
What do I-V graphs show?
An I-V characteristic graph plots current against potential difference for one component. Its shape tells you how the resistance behaves.
| Component | I-V graph | What happens to resistance |
|---|---|---|
| Fixed resistor | Straight line through the origin | Constant |
| Filament lamp | Curve that flattens | Increases as the filament gets hotter |
| Diode | Zero in reverse, steep rise above about 0.6 V forward | Very high in reverse, low in forward |
| Thermistor (NTC) | Curve that steepens as it heats | Decreases as temperature rises |
| LDR | Steeper in brighter light | Decreases as light intensity rises |
In the lamp, metal ions in the hot filament vibrate more and collide with moving electrons more often, so the resistance climbs. A diode acts as a one-way valve, so it is used to protect circuits and to convert alternating current to direct current.
How do thermistors and LDRs work?
A thermistor (negative temperature coefficient, NTC) releases more charge carriers when heated, so its resistance falls. A light-dependent resistor (LDR) does the same in brighter light. Because the resistance changes, the current in a circuit changes, and that signal can switch on a fire alarm, a heater or a street lamp.
A thermistor has resistance 2.0 kΩ and is connected across a 6.0 V supply. Calculate the current.
R = 2000 Ω. I = V ÷ R = 6.0 ÷ 2000 = 0.0030 A = 3.0 mA. If it gets hotter, R falls and the current rises.
Forgetting to convert kΩ to Ω or mA to A. A resistance of 2.0 kΩ is 2000 Ω, and 3 mA is 0.003 A.
Cover the quantity you want in the triangle V over I × R: cover V to see I × R, cover I to see V ÷ R, cover R to see V ÷ I.
- V = I × R; R = V ÷ I; I = V ÷ R.
- 1 volt = 1 joule per coulomb; energy W = Q × V.
- Ohm’s law: I is proportional to V at constant temperature.
- Filament lamp resistance rises with temperature; a diode conducts one way only.
- NTC thermistor and LDR resistances decrease with temperature and light respectively.
Ohm's law and component characteristics: frequently asked questions
What is Ohm’s law?
For an ohmic component at constant temperature, current is directly proportional to potential difference, so V = I × R with a constant R. The I-V graph is a straight line through the origin.
What is the difference between current and potential difference?
Current is the flow of charge each second, measured in amperes. Potential difference is the energy given to or taken from each coulomb, measured in volts.
Why does the resistance of a filament lamp increase?
As the filament heats, the metal ions vibrate more and collide with electrons more often. This makes it harder for charge to flow, so the resistance rises.
What does a diode do?
A diode lets current pass in one direction only. In the reverse direction its resistance is very large, and in the forward direction it conducts above about 0.6 V.
How does a thermistor change with temperature?
The resistance of an NTC thermistor decreases as its temperature rises. This is used in thermostats and fire alarms.
